The present invention relates generally to the field of ink-jet printing and, more particularly, to the delivery of ink and the control of ink pressures to ink-jet print heads.
Ink-jet technology is relatively well developed. The basics of this technology are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk-Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No 5. (October 1988), Vol. 43, No. 4, (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45. No. 1 (February 1994).
The typical thermal ink-jet print head has an array of precisely formed nozzles attached to a print head substrate that incorporates an array of firing chambers that receive liquid ink (i.e., colorants dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a xe2x80x9cfiring resistorxe2x80x9d, located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the thermal inkjet firing resistor, a small portion of the ink near it vaporizes and ejects a drop of ink from the print head. The nozzles are arranged in a matrix array. Properly sequencing the operation of each nozzle causes characters or images to form on the paper as the print head moves past the paper.
An ink delivery system delivers ink at a slight vacuum, known as a xe2x80x9cback pressurexe2x80x9d, to the print head so that the ink does not leak out of the nozzles. Without such back pressure, the ink may leak or xe2x80x9cdroolxe2x80x9d out of the nozzles and onto the printing medium or into the printer mechanism. This back pressure, however, must be small enough so that when the firing resistors are energized, the resistors can overcome the back pressure and eject ink droplets in a consistent and predictable manner. Typically, this vacuum is approximately two to three inches 0f water below atmospheric pressure or minus two to three inches.
Back pressure regulation has become more critical in recent years because of the evolution in the design of print cartridges. The mass of the moving parts and the volume of ink in motion are being reduced so that simpler drive mechanisms can be used. This reduction in mass has decreased the capacity of the materials around the print head to absorb the heat generated by the firing resistors during operation. The result is that unless the transfer of heat from the firing resistors is carefully managed, the ink and the print head may be subjected to wide fluctuations in temperature. These fluctuations in temperature can also result in wide variations in back pressure as the ink heats and cools. The net result is that all of these changes have a degrading affect on print quality.
Accumulators are widely used in hydraulic systems to smooth out pressure fluctuations and to act as shock absorbers against propagating pressure waves. In these applications a compressible gas such as nitrogen or air is used, and the gas is alternately compressed and decompressed as needed. One such use in an ink-jet printing system is disclosed in US Pat. No. 4,223,323 by Bader et al.
While such accumulators work well in those pressure ranges where the gas can be alternately compressed and decompressed, these systems have little affect where the gas is not compressed.
Briefly and in general terms, an apparatus according to the present invention includes a fluid accumulator forming a portion of the ink containment for a print head. The accumulator changes the volume of the ink containment as the temperature of the ink changes so that the ink remains at substantially constant pressure for delivery to the print head.
In another embodiment, an apparatus according to the present invention includes an ink reservoir containing ink at a pressure P1, an ink-jet print head for printing on a medium with ink at a pressure P2, a pressure regulator connected to both the ink reservoir and the print head so that the regulator receives ink at a pressure P1 from the reservoir and supplies ink at a pressure P2 to the print head, where P1 is larger than P2, and a fluid accumulator operatively connected to the print head so that as the temperature of the ink varies, the ink supplied to the print head remains at substantially constant pressure.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken into conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.